The present invention relates to surgical instruments and procedures; in particular to rotary and nonrotary aspirators capable of removing bone cement from bone canals in the replacement and repair of prosthetic bone implants and to a method for removing bone cement in such procedures.
The use of ultrasonic vibration to enhance the performance of surgical mechanical cutting and forming implements such as saws, knives, and spatulas is generally known in the art (Goliamina, Ultrasonic Surgery, Proceedings of the Eighth Int'l. Cong. on Acoustics, London, 1974 pp. 63-69). East German Patent No. 203,229 discloses an ultrasonically activated knife for general surgical application which is intended to increase both the precision and quality of incisions. The application of mechanical vibration to cutting and parting tools is therefore not new to surgical practice and has, in fact, resulted in the commercial introduction of at least one ultrasonically powered instrument for use in cutting cancellous and cortical bone.
The development of prosthetic joints for the hip, knee, elbow and shoulder has offered another application for ultrasonically vibrating instrumentation. Typically, these artificial joints are cemented into a surgically created cavity in the bone. In the case of the hip, the head and neck of the femur are removed, a cavity is reamed into the shaft and the stem of the implant is cemented into this cavity. The cement used is typically methylmethacrylate, an acrylic thermoplastic. These man made joints have an average life of about 10 years, after which they must be replaced. Usually after this time, either the implant itself loosens in the cement, or the cement becomes partially separated from the bone.
Repair of a prosthetic joint requires that first the implant be removed and then all cement be excavated from the cavity. In most cases, the implant is loose upon presentation. The cement, however, is usually found rigidly adherent to the bone. A number of powered rotary burrs have been developed to assist the surgeon in thoroughly cleaning this cavity. These burrs are effective in abrading the plastic, but, because the plastic bone cement is much harder than the surrounding cortical bone, their proper use requires extensive practice in manipulation. Often the cavity is as much as 10 inches deep. Guiding a high speed rotary burr tip at this distance while avoiding inadvertent contact with bone is very difficult to achieve.
Frequently, the surgeon will resort to fluoroscopy as an aid in ensuring that all the residual cement (which contains radio-opaque material) has been removed. Even under the best of circumstances, however, some damage to adjacent bone is inevitable. Melon-ball bone pockets produced by the soft-seeking burr are a constant concern to the orthopedic surgeon because they weaken the cavity into which a new implant must be introduced.
Of all rotational skeletal attachments, the hip joint, in particular, bears the greatest portion of the human body weight. Inasmuch as the implant procedure itself weakens the femor by creating a cavity in otherwise solid supporting physiological matter, any additional enlargement of the original opening presents a risk of future failure, principally through perforation of the bone itself by the implant when subject to the imposed stress of therapeutic exercise or accident. Quite naturally, no surgeon welcomes a repair that, however expertly performed originally, suffers the limitations of his tools. Because access is restricted, the cement is usually firmly adherent and preservation of the remaining structural integrity of the femor is paramount. Hip revision, as this procedure is known, can require as much as 3 or 4 hours to successfully complete. Much of this time is spent in meticulously removing cement.
Recent advances in the art include ultrasonically vibrating spatulas and styluses to separate the plastic cement from the implant and the bone (Klapper and Caillouette, The Use of Ultrasonic Tools in Revision Arthoplasty Procedures, 20: 3 Contemporary Orthopaedics, pp. 273-279) (March 1990). These advances exploit the inability of plastics, and particularly thermoplastics, to suffer cyclic deformation well. Metals and some ceramics have a crystalline or amorphous molecular structure that does not impede the transmission of sound waves. In plastics, however, sound transmission is always accompanied by the generation of heat. If exposed to sound pressures readily conveyed by metallic structures, such as those employed by ultrasonic dental tools and surgical aspirators, virtually all plastics will rapidly heat, melt and even vaporize.
This susceptibility of thermoplastics to intense vibration occurring at an ultrasonic frequency, is the basis of ultrasonic plastic welding--a process widely used in industry to join molded plastic parts for a variety of uses ranging from toys to household appliances (e.g., Ensminger, Ultrasonics: fundamentals, technology, applications, pp. 462-467 (1988 Marcel Dekker Inc.)). Usually, in this process, two mating halves of a plastic part are placed in contact within a nest that conforms to the surface of one of two parts to be joined. An ultrasonic horn, whose face conforms to the exposed surface of the other part, is then brought into intimate contact, under applied pressure, with the assembly. Vibration of the horn is transmitted to the parts. Although the entire plastic is subject to the vibration, the joint between the halves is structurally much weaker than the otherwise homogeneous portions and softens and melts well before any deformation occurs elsewhere. Once the joint melt occurs, vibration ceases, the melt recrystallizes and the bonded part is removed from the nest. Even the strongest reinforced thermoplastics can be joined in this manner within a few seconds.
Direct application of a vibrating tool will also produce local melting (Klapper and Caillouette, supra). By controlling both the contact pressure and the amplitude of vibration, softening can be modulated so that the cement can be transformed into a putty and gently released either from the implant stem or cortical bone. Because the bone is not plastic, and is, in fact, with the exception of tooth enamel, the best anatomical conductor of sound in the human body, it is not deformed by contact with the stylus. Ultrasonic vibration therefore reverses the effect encountered in the use of rotating burrs. The ultrasonic tip moves into the plastic far more easily than into the bone. Ultrasonic excavation is therefore much more easily controlled, even over a distance of 10 to 12 inches, and risk of inadvertent bone damage is significantly reduced when compared to the performance of instruments such as burrs which abrade rather than melt material.
The transformation of phase produced by vibration, although caused by the heat generated from the large cyclic strain losses in plastic, is far more localized than that produced, for example, by a heated tip such as is described in U.S. Pat. No. 4,873,969 to Huebsch. Because the sound waves rapidly propagate in an approximately spherical pattern, the cyclic stress levels rapidly diminish beyond the point of contact. Consequently, only material with a few millimeters of tip contact is softened or melted. On the other hand, a directly heated tip encounters a thermal sink in the cement which draws the energy into the entire surrounding structure. This situation dictates that inordinate amounts of energy have to applied to obtain local melting. In the process thermal elevation of bone as well as the cement occurs. When working close to the bone, a condition that prevails in the lower one third of the cavity, the temperatures so produced can cause tissue necrosis.
The temperature at the operating tip of an ultrasonic aspirator can be controlled to a degree by a preaspiration device. Such a device is disclosed in U.S. Pat. No. 4,493,694 to Wuchinich and includes a sleeve around a central vibrating aspiration tube and holes in the aspiration tube communicating with the passage defined by the sleeve. Irrigating fluid is supplied through the sleeve to cool the tip and is sucked into the aspirating tube through the small holes.